Redo client internals + gateway

This commit is a rewrite of the client module's internals and the
gateway.

The main benefit of this is that there is either 0 or 1 lock retrievals
per event received, and the ability to utilize the ShardManager both
internally and in userland code has been improved.

The primary rework is in the `serenity::client` module, which now
includes a few more structures, some changes to existing ones, and more
functionality (such as to the `ShardManager`).

The two notable additions to the client-gateway bridge are the
`ShardMessenger` and `ShardManagerMonitor`.

The `ShardMessenger` is a simple-to-use interface for users to use to
interact with shards. The user is given one of these in the
`serenity::client::Context` in dispatches to the
`serenity::client::EventHandler`. This can be used for updating the
presence of a shard, sending a guild chunk message, or sending a user's
defined WebSocket message.

The `ShardManagerMonitor` is a loop run in its own thread, potentially
the main thread, that is responsible for receiving messages over an mpsc
channel on what to do with shards via the `ShardManager`. For example,
it will receive a message to shutdown a single shard, restart a single
shard, or shutdown the entire thing.

Users, in most applications, will not interact with the
`ShardManagerMonitor`. Users using the `serenity::client::Client`
interact with only the `ShardMessenger`.

The `ShardManager` is now usable by the user and is available to them,
and contains public functions for shutdowns, initializations, restarts,
and complete shutdowns of shards. It contains utility functions like
determining whether the `ShardManager` is responsible for a shard of a
given ID and the IDs of shards currently active (having an associated
`ShardRunner`). It can be found on
`serenity::client::Client::shard_manager`.

Speaking of the `ShardRunner`, it no longer owns a clone of an Arc to
its assigned `serenity::gateway::Shard`. It now completely owns the
Shard. This means that in order to open the shard, a `ShardRunner` no
longer has to repeatedly retrieve a lock to it. This reduces the number
of lock retrievals per event dispatching cycle from 3 or 4 depending on
event type to 0 or 1 depending on whether it's a message create _and_ if
the framework is in use. To interact with the Shard, one must now go
through the previously mentioned `ShardMessenger`, which the
`ShardRunner` will check for messages from on a loop.

`serenity::client::Context` is now slightly different. Instead of the
`shard` field being `Arc<Mutex<Shard>>`, it is an instance of a
`ShardMessenger`. The interface is the same (minus losing some
Shard-specific methods like `latency`), and `Context`'s shortcuts still
exist (like `Context::online` or `Context::set_game`). It now
additionally includes a `Context::shard_id` field which is a u64
containing the ID of the shard that the event was dispatched from.

`serenity::client::Client` has one changed field name, one field that is
now public, and a new field. `Client::shard_runners` is now
`Client::shard_manager` of type `Arc<Mutex<ShardManager>>`. The
`Client::token` field is now public. This can, for example, be mutated
on token resets if you know what you're doing. `Client::ws_uri` is new
and contains the URI for shards to use when connecting to the gateway.
Otherwise, the Client's usage is unchanged.

`serenity::gateway::Shard` has a couple of minor changes and many more
public methods and fields. The `autoreconnect`, `check_heartbeat`,
`handle_event`, `heartbeat`, `identify`, `initialize`, `reset`,
`resume`, `reconnect`, and `update_presence` methods are now public. The
`token` structfield is now public. There are new getters for various
structfields, such as `heartbeat_instants` and `last_heartbeat_ack`.
The breaking change on the `Shard` is that `Shard::handle_event` now
takes an event by reference and, instead of returning
`Result<Option<Event>>`, it now returns `Result<Option<ShardAction>>`.

`serenity::gateway::ShardAction` is a light enum determining an action
that someone _should_/_must_ perform on the shard, e.g. reconnecting or
identifying. This is determined by `Shard::handle_event`.

In total, there aren't too many breaking changes that most of userland
use cases has to deal with -- at most, changing some usage of `Context`.
Retrieving information like a Shard's latency is currently not possible
anymore but work will be done to make this functionality available
again.

1 files changed, 1 insertions, 8 deletions

diff --git a/examples/02_transparent_guild_sharding/src/main.rs b/examples/02_transparent_guild_sharding/src/main.rs
index ebe5b3e..c1a5bbb 100644
--- a/examples/02_transparent_guild_sharding/src/main.rs
+++ b/examples/02_transparent_guild_sharding/src/main.rs
@@ -27,14 +27,7 @@ struct Handler;
 impl EventHandler for Handler {
     fn message(&self, ctx: Context, msg: Message) {
        if msg.content == "!ping" {
-            // The current shard needs to be unlocked so it can be read from, as
-            // multiple threads may otherwise attempt to read from or mutate it
-            // concurrently.
-            {
-                let shard = ctx.shard.lock();
-                let shard_info = shard.shard_info();
-                println!("Shard {}", shard_info[0]);
-            }
+            println!("Shard {}", ctx.shard_id);
 
             if let Err(why) = msg.channel_id.say("Pong!") {
                 println!("Error sending message: {:?}", why);